1. Field of the Invention
The present invention relates to a static magnetic refrigerator which has a magnetic shield between a magnet for generating a high-intensity magnetic field and a magnetic working material, and activates the magnetic working material to generate coldness.
2. Prior Art
A magnetic refrigerator is a refrigerator which repeats a magnetization process wherein a magnetic working material disposed in a high-intensity magnetic field is adiabatically magnetized by the magnetic field and a process wherein the magnetic field is shut off quickly and the magnetic working material is adiabatically demagnetized so that the magnetic working material generates coldness in the adiabatic demagnetization process.
In conventional magnetic refrigerators, superconducting coils are widely used for magnets which generate high-intensity magnetic fields. The conventional magnetic refrigerators are generally classified into a static type which repeatedly turns on and off the current of the superconducting coil for forming a magnetic field while the magnetic working material is fixed, and an unstatic type which repeats a magnetization process wherein the magnetic working material is magnetized in the highest intensity magnetic field formed by the superconducting coil and a process wherein the magnetic working material is moved to an almost-zero-intensity magnetic field position away from the coil and demagnetized, while constant current flows in the superconducting coil at all times.
The static magnetic refrigerating method wherein the magnetization and demagnetization of the magnetic working material are repeated while the magnetic working material is fixed is convenient for the heat transfer between the magnetic working material and a heat transfer medium. This method can solve the problems described below relating to the reciprocating and rotating operations of the magnetic working material. However, when repeating the magnetization and demagnetization using a conventional method, it is difficult to turn on and off the current flowing in the superconducting coil which controls the generated magnetic field. In addition, turning on and off the large current causes a large Joule heat loss in the external power supply system for the method. This method is thus not practical for industrial applications.
In the method of using the superconducting coil in the permanent current mode and reciprocating or rotating the magnetic working material between the inside of the high-intensity magnetic field being generated at all times and an almost-zero-intensity magnetic field away from the magnetic field coil, however, if the magnetic working material is moved to a completely-zero-intensity magnetic field away from the magnetic field coil in the demagnetization process, the reciprocating or rotating movement distance of the magnetic working material must be made significantly large. As a result, the size of such a refrigerator is required to be made comparatively large while its refrigerating performance is rather low, and a complicated movement means is necessary. The movement stroke is therefore set at a practically satisfactory value. In this case, however, the demagnetization process ends in a residual magnetic field and the magnetic flux density in the magnetic working material is not zero. Consequently, the cooling efficiency in such a demagnetization process is inevitably lower than that in the demagnetization process conducted in a zero-flux density magnetic field. The deviation from the ideal magnetic Carnot cycle diagram for the magnetic working material becomes large, thereby reducing the efficiency of the conventional magnetic refrigerator.
Furthermore, as friction heat is generated by the movement means of the magnetic working material, the efficiency of the refrigerator is lowered. To use the coldness obtained by the refrigerator in practice, the cooling medium of the refrigerator must be circulated in the magnetization and demagnetization processes. However, it is difficult to produce a means which transfers heat to the reciprocating or rotating magnetic working material by contacting the medium to the magnetic working material and simultaneously selects the supply of the cooling medium in the magnetization process and the supply of the coldness transfer medium in the demagnetization process by switching operation. Furthermore, the leakage of the mediums at the periphery of the magnetic working material cannot be prevented, resulting in a cause for the reduction of the thermal efficiency of the refrigerator.
As a conventional technology wherein the movement stroke of the magnetic working material is shortened and demagnetization is performed in a zero-intensity magnetic field, there is a known method, wherein a superconducting sub-coil disposed coaxially at the proximity of the main superconducting coil for generating a high-intensity magnetic field for magnetization generates an opposite magnetic field which cancels the magnetic field generated by the main superconducting coil to form a zero-intensity magnetic field region by the cancellation at a position very close to the opening of the main superconducting coil and to reciprocally move the magnetic working material between the high-intensity magnetic field of the main superconducting coil and the zero-intensity magnetic field region.
In the case of the above-mentioned static magnetic refrigerator in which the magnetic working material is fixed, a type which uses the superconducting coil in the permanent current mode to magnetize and demagnetize the magnetic working material is the most favorable refrigerator, since it requires no complicated movement means for the magnetic working material and no superconducting coil, and the energy efficiency of the refrigerator is superior. As a prior art which achieves this type of refrigerator, a refrigerator which magnetically shields and demagnetizes the magnetic working material by fixing the magnetic working material outside the opening of the superconducting coil and by using a magnetic shield provided reciprocatively between the superconducting coil and the magnetic working material has been disclosed in the Japanese Patent Publication No. 63-31716. The flat plane of the magnetic shield described in the publication has the shape of a small plate being smaller than the opening surface of the coil. Since the plate-shaped magnetic shield is smaller than the sectional area of the high-intensity magnetic field, no magnetic shield space is formed behind the magnetic shield. It is therefore almost impossible to demagnetize the magnetic working material. This case is explained as follows. If the plate has high-intensity magnetism, the magnetic lines of force simply permeate the plate, and if the plate is a superconducting plate, the magnetic lines of force pass around the plate to its rear side. In other words, a magnetic shield space can be formed behind a plate-shaped magnetic shield only when the surface area of the plate is sufficiently larger than the sectional area of the magnetic field generation source located ahead of the plate.
By a separate application, the inventors of the present invention have already proposed a magnetic refrigerator having a tube-shaped superconducting magnetic shield disposed between the superconducting coil and the magnetic working material to adiabatically magnetize the magnetic working material in the high-intensity magnetic field of the superconducting coil and to adiabatically demagnetize the magnetic working material by inserting or accommodating the magnetic working material into the hollow section of the magnetic shield disposed close to the coil. [Japanese Pat. Appln's Nos. 2-305586 and 3-59637, U.S. patent application Ser. No. 07/788,100, Canadian Pat. Appln (which appln number is still unknown), and European Pat. Appln No. 91202909.7]
With this refrigerator, the magnetic working material accommodated in the hollow section of the magnetic shield can be completely demagnetized by activating the reciprocating means for reciprocating the magnetic working material or the magnetic shield through the use of the fact that a completely-zero-intensity magnetic field can be achieved in the hollow section of the superconducting cylinder even if the magnetic working material is accommodated in a very high-intensity magnetic field capable of achieving a magnetic flux density of 5 T or more. In addition, since the magnetic shield can completely shield the high-intensity magnetic field even if it is disposed at a position close to or at the center of the high-intensity magnetic field coil, the reciprocating stroke can be made shorter. Furthermore, if the magnetic shield is reciprocated, the same result as that described above can be obtained by fixing the magnetic working material in a constant magnetic field which is generated when the superconducting coil is used in the permanent current mode. The cooling operation by the cooling medium and the circulation of the cooling medium can thus be extremely simplified and the above-mentioned problems caused by the conventional method can be solved.
Even in the method of using the cylindrical magnetic shield, however, a reciprocating means is still necessary although the stroke of the movement is short. Moreover, by fixing the magnetic working material and by moving the magnetic shield instead of the magnetic working material, the performance of the static magnetic refrigerator can be made superior. However, this case also causes problems; for example, large force is necessary to reciprocate the magnetic shield in a high-intensity magnetic field.